`pip_compile`: Prefer `uv export` over `uv pip compile` to respect default `[dependency-groups]`

Given a `pyproject.toml` with [dependency-groups](https://docs.astral.sh/uv/concepts/projects/dependencies/#dependency-groups), eg
```
[dependency-groups]
group-a = [
"requests"
]
group-b = []

[tool.uv]
default-groups = ["group-a"]
```

* `uv export` respects the `default-groups` and produces a `requirements.in` containing `requests` and transitive dependencies
* `uv pip compile` on the other hand generates an empty file.

The `pip_compile` rule is using `uv pip compile` and hence misses the groups.

Start by locating the implementation of the `pip_compile` rule and compare its current `uv pip compile` invocation with the reported `uv export` behavior. Verify the example `pyproject.toml` and confirm that the completed change includes the default dependency group's packages and transitive dependencies in the generated requirements output.

Written by the indexing model from the issue text.